I don't know which would be "better" between the blue or purple acintics, but I can tell you the difference is the "rating" on the bulb. The purple is 420 NM and the blue is 460 NM.

I have a Current-USA light fixture with SunPaq bulbs that came with my tank and I like it well enough. For more info on SunPaq bulbs check out Current's site: http://www.current-usa.com/sunpaq.html